CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 26,184	$ 746	$ 5,504	$ 6,804	$ 21,292	$ 4,129	$ 6,128	$ 14,654	$ 39,238	$ 46,203	$ 50,162
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities, net of defered income tax expense (benefit) of $14,688, $(2,092) and $(7,021)									27,278	(3,885)	(13,037)
Reclassification adjustment for net realized investment gains included in net income, net of income tax expense of $(4,483), $(3,628) and $(2,668)									(8,326)	(6,736)	(4,956)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income, net of deferred income tax expense of $(587), $(457) and $(693):
Net actuarial loss									958	1,549	863
Prior service credit									(2,047)	(2,399)	(2,150)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(1,089)	(850)	(1,287)
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income tax expense (benefit) of $1,507, $(474) and $(2,126):
Net actuarial gain (loss)									5,571	(542)	(3,637)
Prior service credit (cost)									96	(339)	(312)
Total change in funded status of affiliate's pension and postretirement benefit plans									5,667	(881)	(3,949)
Other comprehensive income (loss)									23,530	(12,352)	(23,229)
Total comprehensive income									$ 62,768	$ 33,851	$ 26,933